03-Presentation Directional Protection

Application of Directional Overcurrent and Earthfault Protection

Application of Directional Overcurrent and Earthfault Protection - January 2004

Directional Protection

Application of Directional Overcurrent and Earthfault Protection - January 2004

Need for Directional Control Generally required if current can flow in both directions through a relay location e.g. Parallel feeder circuits Ring Main Circuits

0.9

0.1

0.5

0.5

0.1

0.9

Relays operate for current flow in direction indicated. (Typical operating times shown).

Application of Directional Overcurrent and Earthfault Protection - January 2004

Ring Main Circuit With ring closed : Both load and fault current may flow in either direction along feeder circuits. Thus, directional relays are required. Note: Directional relays look into the feeder. Need to establish principle for relay.

51

67

67

67

Load

51

67

Load

67

Load Application of Directional Overcurrent and Earthfault Protection - January 2004

67

Ring Main Circuit Procedure : 1. Open ring at A Grade : A' - E' - D' - C' - B' 2. Open ring at A' Grade : A - B - C - D - E Typical operating times shown. Note : Relays B, C, D’, E’ may be non-directional. A

B'

B

C'

C

0.1

1.3

0.5

0.9

1.7

A'

E

E'

0.1

1.3

1.7

Application of Directional Overcurrent and Earthfault Protection - January 2004

0.9

D'

0.5

D

Ring System with Two Sources Discrimination between all relays is not possible due to different requirements under different ring operating conditions.

}

For F1 :- B’ must operate before A’ For F2 :- B’ must operate after A’

Not Compatible B

F1 B'

A

B

C'

C

A' F2 Application of Directional Overcurrent and Earthfault Protection - January 2004

D

D'

Ring System with Two Sources Option 1 Trip least important source instantaneously then treat as normal ring main. Option 2 Fit pilot wire protection to circuit A - B and consider as common source busbar. B

A

Option 1

50

Option 1

PW

PW

Option 2

Option 2

Application of Directional Overcurrent and Earthfault Protection - January 2004

Option 1

Parallel Feeders Non-Directional Relays :‘F’

51 A

51 C

51 B

51 D

“Conventional Grading” :Grade ‘A’ with ‘C’ and Grade ‘B’ with ‘D’

Load

A&B C&D

Relays ‘A’ and ‘B’ have the same setting. Fault level at ‘F’ Application of Directional Overcurrent and Earthfault Protection - January 2004

Parallel Feeders Consider fault on one feeder :I1 + I2 I1

51 A

I2

51 B

C

51

D

51

LOAD

Relays ‘C’ and ‘D’ see the same fault current (I2). As ‘C’ and ‘D’ have similar settings both feeders will be tipped. Application of Directional Overcurrent and Earthfault Protection - January 2004

Parallel Feeders Solution:- Directional Control at ‘C’ and ‘D’ I1 + I2 I1

51 A

C I2

51 B

D

67

LOAD

67

Relay ‘D’ does not operate due to current flow in the reverse direction.

Application of Directional Overcurrent and Earthfault Protection - January 2004

Parallel Feeders Setting philosophy for directional relays E 51 A

C

Load

67 51

51 B

D

67

Load current always flows in ‘non-operate’ direction. Any current flow in ‘operate’ direction is indicative of a fault condition. Thus Relays ‘C’ and ‘D’ may be set :- Sensitive (typically 50% load) - Fast operating time (i.e. TMS=0.1) Application of Directional Overcurrent and Earthfault Protection - January 2004

Parallel Feeders

Usually, relays are set :50% full load current (note thermal rating) Minimum T.M.S. (0.1) Grading procedure :1. Grade ‘A’ (and ‘B’) with ‘E’ assuming one feeder in service. 2. Grade ‘A’ with ‘D’ (and ‘B’ with ‘C’) assuming both feeders in service.

Application of Directional Overcurrent and Earthfault Protection - January 2004

Parallel Feeders - Application Note

Grade B with C at If1 Grade B with D at If2 (in practice) A Grade A with B at If Load - but check that sufficient margin exists for bus fault at Q when relay A sees total fault current If2, but relay B sees only If2/2.

If2

P B

D

Load

C B

If1:One Feeder If2:Two Feeders

D

D

C

B

A M = Margin If2

If2/2

M

M

If2/2 If1If2

Application of Directional Overcurrent and Earthfault Protection - January 2004

Q

M

If

Establishing Direction

Application of Directional Overcurrent and Earthfault Protection - January 2004

Establishing Direction:- Polarising Quantity

The DIRECTION of Alternating Current may only be determined with respect to a COMMON REFERENCE. In relaying terms, the REFERENCE is called the POLARISING QUANTITY. The most convenient reference quantity is POLARISING VOLTAGE taken from the Power System Voltages.

Application of Directional Overcurrent and Earthfault Protection - January 2004

Directional Decision by Phase Comparison (1) S1 = Reference Direction = Polarising Signal = VPOL S2 = Current Signal = I OPERATION when S2 is within ±90° of S1 :S1 S2

S2

S2

S2

Application of Directional Overcurrent and Earthfault Protection - January 2004

S2

S2

S2

Directional Decision by Phase Comparison (2) RESTRAINT when S2 lags S1 by between 90° and 270° :S1

S2

S2

S2

S2

S2

S2 S2

Application of Directional Overcurrent and Earthfault Protection - January 2004

Polarising Voltage for ‘A’ Phase Overcurrent Relay

OPERATE SIGNAL

=

POLARISING SIGNAL :-

Application of Directional Overcurrent and Earthfault Protection - January 2004

IA Which voltage to use ? Selectable from VA VB VC VA-B VB-C VC-A

Directional Relay Applied Voltage Applied Current

: :

VA IA VA IA

Operate IAF VAF

Restrain

Question : - is this connection suitable for a typical power system ? Application of Directional Overcurrent and Earthfault Protection - January 2004

Polarising Voltage Applied Voltage : VBC Applied Current : IA VA IA IAF MAXIMUM SENSITIVITY LINE

VBC IVBC

ØVBC ZERO SENSITIVITY LINE

X Polarising voltage remains healthy X Fault current in centre of characteristic

Application of Directional Overcurrent and Earthfault Protection - January 2004

Relay Connection Angle The angle between the current applied to the relay and the voltage applied to the relay at system unity power factor e.g. 90° (Quadrature) Connection :

IA and VBC

IA VA

90° VBC

VC

VB

The 90° connection is now used for all overcurrent relays. 30° and 60° connections were also used in the past, but no longer, as the 90° connection gives better performance. Application of Directional Overcurrent and Earthfault Protection - January 2004

Relay Characteristic Angle (R.C.A.) for Electronic Relays The angle by which the current applied to the relay must be displaced from the voltage applied to the relay to produce maximum operational sensitivity e.g. 45° OPERATE

RESTRAIN

IA FOR MAXIMUM OPERATE SENSITIVITY

VA

45°

RCA

Application of Directional Overcurrent and Earthfault Protection - January 2004

VBC

90° Connection - 45° R.C.A.

MAX SENSITIVITY LINE

OPERATE

IA VA

RESTRAIN

IA FOR MAX SENSITIVITY

VA 45°

90°

45° VBC

VC

135°

VB

RELAY CURRENT VOLTAGE A

IA

VBC

B

IB

VCA

C

IC

VAB

Application of Directional Overcurrent and Earthfault Protection - January 2004

VBC

90° Connection - 30° R.C.A. OPERATE MAX SENSITIVITY LINE IA FOR MAX SENSITIVITY

RESTRAIN

IA VA

VA 30°

90° VBC

30° 150°

VC

VB

RELAY CURRENT VOLTAGE A

IA

VBC

B

IB

VCA

C

IC

VAB

Application of Directional Overcurrent and Earthfault Protection - January 2004

VBC

Selection of R.C.A. (1) Overcurrent Relays 90° connection 30° RCA (lead) Plain feeder, zero sequence source behind relay

Application of Directional Overcurrent and Earthfault Protection - January 2004

Selection of R.C.A. (2) 90° connection 45° RCA (lead) Plain or Transformer Feeder :- Zero Sequence Source in Front of Relay

Transformer Feeder :- Delta/Star Transformer in Front of Relay

Application of Directional Overcurrent and Earthfault Protection - January 2004

Directional Earthfault Protection

Application of Directional Overcurrent and Earthfault Protection - January 2004

Directional Earth Fault Requirements are similar to directional overcurrent i.e. need operating signal and polarising signal Operating Signal obtained from residual connection of line CT's i.e. Iop = 3Io Polarising Signal The use of either phase-neutral or phase-phase voltage as the reference becomes inappropriate for the comparison with residual current. Most appropriate polarising signal is the residual voltage. Application of Directional Overcurrent and Earthfault Protection - January 2004

Residual Voltage May be obtained from ‘broken’ delta V.T. secondary. A B C VA-G

VB-G VC-G

VRES = VA-G + VB-G + VC-G = 3V0

VRES

Notes : 1. VT primary must be earthed. 2. VT must be of the '5 limb' construction (or 3 x single phase units) Application of Directional Overcurrent and Earthfault Protection - January 2004

Directional Earth Fault Relays

Relay Characteristic Angle 0 - Resistance earthed systems 45 (I lags V) - Distribution systems (solidly earthed) 60 (I lags V) - Transmission systems (solidly earthed)

Application of Directional Overcurrent and Earthfault Protection - January 2004

Residual Voltage Solidly Earthed System

E

S

F

R ZL

ZS

A-G VA VA

VB VC

VC VA

VB VC

VB VC

VRES VA VC

VB

VRES VB

VB VC

Residual Voltage at R (relaying point) is dependant upon ZS / ZL ratio.

Z S0 VRES= x 3E 2Z S1 + Z S0 + 2Z L1 + ZL0 Application of Directional Overcurrent and Earthfault Protection - January 2004

Residual Voltage Resistance Earthed System S

E

R

ZS

N

F

ZL

ZE

A-G

G VA-G G.F

VC-G

VB-G VC-G

VRES VA-G VC-G

VRES=

S R G.F

S V A-G R G.F

S

VB-G

VRES VA-G VC-G

VB-G VC-G

VB-G

VB-G

VRES

VB-G

VC-G

Z S0 + 3Z E x 3E 2Z S1 + Z S0 + 2Z L1 + ZL0 + 3Z E

Application of Directional Overcurrent and Earthfault Protection - January 2004

Directional Earth Fault Relays Relay Characteristic Angle 0 - Resistance earthed systems 45 (I lags V) - Distribution systems (solidly earthed) 60 (I lags V) - Tranmission systems (solidly earthed) Zero sequence network :ZS0

3R

I0

V0

V0 = ( - ) I0 (ZS0 + 3R) Application of Directional Overcurrent and Earthfault Protection - January 2004

ZL0

Current Polarising A solidly earthed, high fault level (low source impedance) system may result in a small value of residual voltage at the relaying point. If residual voltage is too low to provide a reliable polarising signal then a current polarising signal may be used as an alternative. The current polarising signal may be derived from a CT located in a suitable system neutral to earth connection. e.g.

OP POL DEF Relay

Application of Directional Overcurrent and Earthfault Protection - January 2004

Directional Control Static Relay (METI + MCGG)

M.T.A. Selectable

Characteristic Selectable

I

51

V

67 Overcurrent Unit (Static)

Application of Directional Overcurrent and Earthfault Protection - January 2004

Directional Unit (Static)

I

Numerical Relay Directional Characteristic

Characteristic angle Øc Øc = -180° --- 0° --- + 180° in 1° steps

Zone of forward start forward operation +Is

Øc - 90° Polarising thresholds Vp > 0.6V Vop > 0.6 to 80V in 0.2V steps for example Application of Directional Overcurrent and Earthfault Protection - January 2004

Øc

Reverse start

Øc + 90° -Is